ORGANIC PARTNERSHIPS OR SYMBIOSIS 



173 



peculiar structures, and, as they are of no direct use to the tree, we 

 must say that it has produced them only for the ants. Here, again, 

 natural selection must have gradually brought about the development 

 of these nutritive cushions, though as yet we do not know what the 

 beginnings of the process may have been. In no case can the origin of 

 these cushions be referred to any direct influence of the environmental 

 conditions. 



We may now pass to the association of two species of plants, of 

 which the lichens furnish the best-known and probably most complete 

 illustration. Till about twenty years ago the lichens, which in so 

 many diverse forms clothe the bark of trees, the stones, and the rocks, 

 were regarded as simple plants like the flowering plants, the ferns, 

 or the mosses ; and many lichenologists occupied themselves with the 



Pia. 38. A fragment of a lichen {Epheie kerneri), magnified 450 times. 

 a, the green alga-cells. P, the fungoid filaments. After Kerner. 



exact systematic distinction of about a thousand species, each of 

 which could be as well and exactly classified, according to form, 

 colour, habitat, and minute structure, as any other kind of plant. 

 Then De Bary and Schwendener discovered that the lichens were 

 made up of two kinds of plants, fungi and algae, so intimately 

 associated with and adapted to one another, that on coming together 

 they always assume the same specific form. 



The framework, and therefore the largest part, and the one 

 which determines the form of a lichen, is due to the fungus (Fig. 38). 

 Colourless threads of fungus ramify in a definite manner according 

 to the species of fungus, and in the network of spaces left by this 

 ramification green alga-cells (a) lie singly, or in rows, or groups. 

 The fungus is propagated by multitudes of minute spores, which it 



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